The epidermis, the squamous stratified epithelium of the skin, consists of multiple sublayers and is one of the most important barriers of the body with the outside world. Stratum corneum is the outermost layer of the epidermis and the final anucleated step in keratinocyte differentiation from the cells in nucleated epidermal layers. Although stratum corneum is recognised as the most important physical barrier, the nucleated epidermal layers are also significant in barrier function as evidenced by studies with burn victims (1, 2). Together, the skin barrier protects against extensive water loss in one direction (inside-outside barrier) and against the invasion of harmful substances from the environment (outside-inside barrier) (2). The maintenance of the barrier is also important for balanced proliferation in the basal layer and preservation of the calcium ion gradient and thus proper epidermal differentiation (3).
In the mammalian epidermis, tight junctions (TJ) are mainly found in the stratum granulosum, or the granular layer of the epithelium, located underneath the stratum corneum (4). TJ are dynamic cell-cell junctions that connect neighbouring cells, control the paracellular pathway of molecules acting as barriers, and have a fence function by separating apical cell membranes from basolateral cell membranes (5). They are important not only in inside-outside barrier function but also in outside-inside barrier function.
Probiotics have been defined as “live microorganisms, which when administered in adequate amounts, confer health benefit on the host” (13). Research of the effects of oral administration of probiotics on the epidermis has previously been undertaken. Probiotics have been suggested to modulate the immunity systemically and have been suggested for prophylactic use to alleviate atopic eczema in children (17).
Few studies have been undertaken to study how probiotics affect keratinocytes when administered topically. However, in one study, sonicated lysates of Streptococcus thermophilus have been observed to increase the level of ceram ides in stratum corneum in vitro and in vivo (22).